1. Field of the Invention
This invention relates to a method and system for coupling electromagnetic energy from an active device to current return paths in a manner that reduces emanated noise current by returning noise current to a common ground of the generating active device.
2. Description of the Related Art
Self-contained electronic systems, in particular systems housed in metal cases, have active devices mounted on printed circuit boards (PCB). PCBs often act as common ground for such active devices. Common ground provides for current generated by active devices to return or complete a current loop back to current generating devices. Examples of self contained electronic systems include personal computers (PC), high-end audio/video devices, and electrical test equipment. Active devices in these systems include very large-scale integrated (VLSI) circuits having a number of switching transistors. Such VLSI components generate a measurable amount of electromagnetic energy, in particular noise current. An amount of noise current is directly returned back to the active device. Some noise current is radiated and returned to the PCB which holds the active device. Further, an amount of the noise current radiates into free space between the metal case and the active device. Noise current that is not returned directly back to the active device nor returned back to the PCB, either is returned through the metal case or is returned to common ground by way of adjacent devices. Any noise current that is not directly returned to the active device can lead to an increase in electromagnetic interference (EMI). EMI is strictly regulated by agencies such as the Federal Communication Commission. The radiated noise current that is returned by way of adjacent devices can lead to faulty operation of adjacent devices, including data processing errors.
Referring now to FIG. 1, a block diagram illustrates a multi-device electronic system. Active devices 100, 105, and 110 are mounted on a PCB 115. PCB 115 acts as common ground. Active devices 100, 105, and 110 generate noise current 120. Noise current 120 completes a current loop directly back to respective noise current generating active devices 100, 105, and 110. Noise current 125 is radiated from respective devices 100, 105, and 110 and is returned back to common ground PCB 115. Noise current 130 is current that is radiated from the respective active devices 100, 105, and 110 that is not returned directly to the originating device nor is returned directly to PCB 115. Noise current 130 returns to common ground PCB 115 by way of metal case 140 or by way of adjacent devices.
Referring now to FIG. 2, an electrical circuit illustrates current that is generated by an active device in a multi-device system. The active device acts as voltage source 200. Radiation resistance 220 is in series with voltage source 200. In parallel with voltage source 200 are impedances Z1205, Z2210, and Z3215. Impedance Z1205 represents the impedance of current that directly loops back to voltage source 200; this current is represented as current noise 120 of FIG. 1, and is current that does not leave device 100. Impedance Z2210 represents the impedance of the current that loops back to voltage source by way of common ground. Common ground is represented by PCB 115, and the current is represented by current noise 125. Impedance Z3215 represents the impedance of the current that travels through air and is directed back to common ground by way of a metal case or by way of adjacent devices; this current is represented as current noise 130 of FIG. 1.
In an effort to eliminate active device generated noise current from affecting adjacent devices, implementation has been made of shields, shielding devices, and cans (shields or cans). Such shields and cans are placed over individual active devices and act as conductive return paths that provide current return loops to the PCB common ground and to the originating active device.
Now referring to FIG. 3, a block diagram illustrates a multi-device electronic system implementing shielding structures. Active devices 100, 105, and 110 are noise current generating devices. A shield or a can 300 is placed over device 100; a shield or a can 305 is placed over device 105; and a shield or a can 310 is placed over device 110. Shields and cans 300, 305, and 310 contain the current within their structure, effectively making a very short current return path to the originating active device. Shields and cans 300, 305, and 310 effectively decrease the distance xe2x80x9cdxe2x80x9d of the parallel plate capacitance equation with a result of increasing capacitance, in effect increasing coupling and providing a return path to the originating active device. The result is the drawing of noise current back to the originating noise current device by way of shields and cans 300, 305, and 310. Radiated noise current 130 is greatly reduced or eliminated, with noise current 130 returned back to the respective originating active device.
These practices of enclosing active devices; however, have drawbacks and problems, including the impracticability of completely enclosing the active device, due to external connections, ventilation requirements, serviceability needs, and fabrication feasibility.
Adding a localized shield or can over an active device requires additional engineering design. A shield or can potentially reduces or cuts off air-flow to the device leading to the risk of overheating. Further a can requires space on PCBs for connection and adds manufacturing steps. Shields or cans add difficulty to servicing the device, and because of their conductive nature, risk creating short-circuits on the product. Further, shields or cans are dependent upon preexisting, and potentially inadequate, current return paths of the system.
What is needed and is disclosed herein is an invention that provides for a method and a system to effectively reduce radiated electrical magnetic energy from affecting devices in a multi-device electronic system, and to provide adequate current return path to active devices that originate the current.
In one embodiment of the invention, a dielectric structure is provided to a noise current generating device where the dielectric structure and the noise generating device are connected to a common ground, and radiated noise current follows a return path from the dielectric structure to the common ground.
Certain embodiments of the invention provide common dielectric structures for a number of noise generating devices, and individual common dielectric structures for individual noise generating devices.
In other certain embodiments of the invention, dielectric structures are placed directly onto noise generating devices. Alternatively in other embodiments, dielectric structures and noise generating devices have a certain amount of space between them.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.